Results from three published clinical trials have shown that screening for occult blood in the stool reduces the death rate from colorectal cancer. The reduction in mortality is a result of early detection and subsequent removal of malignant tumors before they metastasize and, possibly, the detection and removal of premalignant adenomatous polyps. Results from the Minnesota Colon Cancer Control Study showed that, during a 13-year period, the incidence of colorectal cancer was 12 percent lower in persons who had fecal occult blood screening than in persons not so screened. The investigators recently reported their data on the cumulative incidence of colorectal cancer after 18 years of follow-up.

The Minnesota study, started in 1975, evaluated the effectiveness of fecal occult blood testing in reducing the rate of death from colorectal cancer. Adults between 50 and 80 years of age were recruited for the study and randomly assigned to annual-screening, biennial-screening, or no-screening (control) groups. The participants were asked to prepare two guaiac-impregnated paper slides for each of three consecutive stools and to submit the samples. Persons with at least one positive stool card were offered further diagnostic testing that most commonly included barium enema, flexible sigmoidoscopy or full colonoscopy. The screening tests were completed in 1992 and all participants have been followed since that time.

There were 22,323 men and 24,122 women enrolled in the study. Those randomized to the annual-screening and biennial-screening groups were offered 11 and six separate opportunities for screening, respectively; the rate of compliance with screening was 75 and 78 percent, respectively. Of the participants who had at least one positive stool card, 83 percent in the annual-screening group and 84 percent in the biennial-screening group underwent further diagnostic evaluation with colonoscopy or a combination of flexible sigmoidoscopy and double-contrast barium enema. Follow-up for vital status was more than 91 percent in all three groups through year 18 of the study, with death certificates reviewed on all but three of 15,783 persons who died during this time.

Through year 18 of the study, 417 new cases of colorectal cancer were diagnosed in the annual-screening group, 435 in the biennial-screening group and 507 in the control group. The ratios of cumulative incidence rates in the two screening groups were 0.8 and 0.83, respectively, compared with the control group. In the group screened annually, the positive predictive value for one guaiac-positive slide was 0.87 percent and increased to 4.53 percent for six positive slides. The percentages were similar in the biennial-screening group. For adenomatous polyps at least 1 cm in diameter, there was an association between the number of positive slides and the positive predictive value of the tests in both screening groups.

The authors conclude on the basis of this data that the use of fecal occult blood testing on an annual or biennial basis reduces the risk of colorectal cancer. Early detection and treatment of this disease reduces morbidity and mortality.